Fetch stage handling of indirect jumps in a processor pipeline

ABSTRACT

Systems and methods are disclosed for fetch stage handling of indirect jumps in a processor pipeline. For example, a method includes detecting a sequence of instructions fetched by a processor core, wherein the sequence of instructions includes a first instruction, with a result that depends on an immediate field of the first instruction and a program counter value, followed by a second instruction that is an indirect jump instruction; responsive to detection of the sequence of instructions, preventing an indirect jump target predictor circuit from generating a target address prediction for the second instruction; and, responsive to detection of the sequence of instructions, determining a target address for the second instruction before the first instruction is issued to an execution stage of a pipeline of the processor core.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application No.63/002,307, filed on Mar. 30, 2020, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

This disclosure relates to fetch stage handling of indirect jumps in aprocessor pipeline.

BACKGROUND

In order to increase performance pipelined processors may include anindirect jump target predictor that generates predictions of targetaddresses of indirect jump instructions, which may depend on data thatmay not become available until the indirect jump instruction reaches alater stage of a processor pipeline. The target address predictions maybe used to fetch upcoming instructions while waiting for the indirectjump instruction to pass through the pipeline and be retired.Mispredictions of the target addresses may cause problems, includingperformance penalties and pollution of the state of the indirect jumptarget predictor.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is best understood from the following detaileddescription when read in conjunction with the accompanying drawings. Itis emphasized that, according to common practice, the various featuresof the drawings are not to-scale. On the contrary, the dimensions of thevarious features are arbitrarily expanded or reduced for clarity.

FIG. 1 is a block diagram of an example of an integrated circuit forexecuting instructions using fetch stage handling of indirect jumps in aprocessor pipeline.

FIG. 2 is a block diagram of an example of a portion of a processorpipeline for executing instructions using fetch stage handling ofindirect jumps.

FIG. 3 is a memory map of an example of a sequence of instructions thatincludes a first instruction with a result that depends on an immediatefield of the first instruction and a program counter value followed by asecond instruction that is an indirect jump instruction.

FIG. 4 is a flow chart of an example of a process for fetch stagehandling of indirect jumps.

FIG. 5 is a flow chart of an example of a process for determining atarget address for an indirect jump instruction that depends on aprogram counter and one or more immediates of a sequence ofinstructions.

FIG. 6 is a flow chart of an example of a process for selectivelydisabling an indirect jump target predictor circuit in the absence ofindirect jumps.

DETAILED DESCRIPTION Overview

Described herein are systems and methods for fetch stage handling ofindirect jumps in a processor pipeline. In some processor architecturesa sequence of instructions including an indirect jump instruction can beused to specify a target address in a large virtual address space. Anearlier instruction in the sequence can add an immediate to value to aprogram counter value. The result can then be added to a secondimmediate included in the indirect jump instruction and shifted to allowa large range of jumps in relation to the program counter value. Thissequence of instructions forms an immediate jump for which the targetaddress can be determined based on immediates and a program countervalue, which is information that will be available in a fetch stage of aprocessor pipeline. For example, in the RISC-V instruction set, asequence of instructions including an AUIPC instruction followed by aJALR instruction form an immediate jump. However, an indirect jumptarget predictor circuit may generate a target address prediction forthe indirect jump instruction of the sequence of instructions. This maywaste power in the indirect jump target predictor circuit and may alsoresult in occasional mispredictions of target addresses that causeperformance penalties and/or pollute the predictor state of the indirectjump target predictor circuit.

Some implementations, solve or mitigate these problems by addingcircuitry to a processor core to detect these sequences of instructionsforming an immediate jump and determining the target address of theindirect jump of the sequence of instructions in a fetch stage of aprocessor pipeline. For example, the determined target address may beinserted in a fetch target queue and used in lieu of a target addressprediction from an indirect jump target predictor circuit of theprocessor core. In some implementations, the indirect jump targetpredictor circuit may be disabled when the indirect jump instruction ofthe sequence of instructions is being fetched to prevent the indirectjump target predictor circuit from wasting power generating a targetaddress prediction for the indirect jump instruction. For example, thesequence of instructions may be detected in an earlier stage of apipeline with multiple fetch stages, such as when a cacheline ofinstructions s being loaded into an L1 instruction cache. This earlydetection may allow an immediate jump hint to be generated that will beavailable early enough to control an enable input of the indirect jumptarget predictor circuit as the sequence of instructions is being readout of the L1 instruction cache and disable the indirect jump targetpredictor circuit.

Another technique for reducing power consumption in an indirect jumptarget predictor circuit is to detect the presence or absence ofindirect jump instructions in a cacheline as the cacheline is beingloaded into an L1 instruction cache to generate and indirect jump hintthat can be used in a later fetch stage of a processor pipeline toenable or disable the indirect jump target predictor circuit. That is,if no indirect jump instruction is detected in the cacheline, thedisable the indirect jump target predictor circuit when reading outinstructions from the cacheline. This technique may be combined with theimmediate jump handling described above and elsewhere herein. Forexample, an enable input of an indirect jump target predictor circuitmay be set to an inactive level if an indirect jump hint indicates theabsence of indirect jump instructions or an immediate jump hintindicates the presence of a sequence of instructions forming animmediate jump.

In some implementations, the techniques for fetch stage handling ofindirect jumps in a processor pipeline may be used to realize one ormore advantages over conventional processors. For example, thestructures and techniques described herein may reduce power consumptionin a processor core, reduce pollution of a predictor state of anindirect jump target predictor circuit, and/or improve performance of aprocessor core.

As used herein, the term “circuit” refers to an arrangement ofelectronic components (e.g., transistors, resistors, capacitors, and/orinductors) that is structured to implement one or more functions. Forexample, a circuit may include one or more transistors interconnected toform logic gates that collectively implement a logical function.

Details

FIG. 1 is a block diagram of an example of an integrated circuit 110 forexecuting instructions using fetch stage handling of indirect jumps in aprocessor pipeline. The integrated circuit 110 includes a processor core120. The processor core 120 includes a processor pipeline 130 thatincludes an indirect jump target predictor circuit 132 configured togenerate predictions for target addresses of fetched indirect jumpinstructions. The processor core 120 includes one or more register files140 that include a program counter 142. The processor core 120 includesan L1 instruction cache 150 and an L1 data cache 152. The integratedcircuit 110 includes an outer memory system 160, which may includememory storing instructions and data and/or provide access to a memory162 external to the integrated circuit that stores instructions and/ordata. The processor core 120 includes an immediate jump handler circuit170, which may be configured to detect sequences of instructionsincluding a indirect jump instruction that has a target address that canbe determined based on information available in a fetch stage of thepipeline 130, and determine this target address in a fetch stage of thepipeline to be used in lieu of a prediction of the target address. Theprocessor core 120 includes an indirect jump detector circuit 180, whichmay be configured to check for indirect jump instructions in cachelinesas they are loaded into the L1 instruction cache 150 and disable theindirect jump target predictor circuit 132 when cachelines withoutindirect jumps are read from the L1 instruction cache 150 to save power.The integrated circuit 110 may provide advantages over conventionalprocessor architectures, such as, for example, avoiding mispredictionsof target addresses and resulting pollution of an indirect jumppredictor and performance degradation, and/or conservation of powerconsumption. For example, the integrated circuit 110 may implement theprocess 400 of FIG. 4. For example, the integrated circuit 110 mayimplement the process 600 of FIG. 6.

The integrated circuit 110 includes a processor core 120 including aprocessor pipeline 130 configured to execute instructions. The pipeline130 includes one or more fetch stages that are configured to retrieveinstructions from a memory system of the integrated circuit 110. Forexample, the pipeline 130 may fetch instructions via the L1 instructioncache 150. For example, the pipeline 130 may include the processorpipeline 200 of FIG. 2. The pipeline 130 may include additional stages,such as decode, rename, dispatch, issue, execute, memory access, andwrite-back stages. For example, the processor core 120 may include apipeline 130 configured to execute instructions of a RISC V instructionset.

The integrated circuit 110 includes an indirect jump target predictorcircuit 132 in a fetch stage of the pipeline 130 configured to generatepredictions for target addresses of fetched indirect jump instructions.For example, the indirect jump target predictor circuit 132 may be theindirect jump target predictor circuit 220 of FIG. 2. For example, theindirect jump target predictor circuit 132 may output the predictions toa fetch target queue.

The indirect jump target predictor circuit 132 is a structure used topredict the target of indirect jump instructions (e.g., RISC-V JALRinstructions). For example, the indirect jump target predictor circuit132 may be an ITTAGE-style predictor, which is similar in design to abranch direction predictor (BDP). However, as opposed to predictingbranch direction, the indirect jump target predictor circuit 132provides target addresses. For example, the indirect jump targetpredictor circuit 132 may be SRAM-based, and to be more area efficientmay be designed to use single-ported memories. In some implementations,there is no structural hazard between prediction and updates on theindirect jump target predictor circuit 132.

The integrated circuit 110 includes one or more register files 140 thatinclude a program counter 142 for the processor core 120. For example,the program counter 142 may be stored in a register. For example, theprogram counter 142 may be stored using a program counter map table thatis used to keep track of program counter for instructions in a reorderbuffer window.

The integrated circuit 110 includes an L1 instruction cache 150 for theprocessor core 120. The L1 instruction cache 150 may be aset-associative cache for instruction memory. To avoid the long latencyof reading a tag array and a data array in series, and the high power ofreading the arrays in parallel, a way predictor may be used. The waypredictor may be accessed in an early fetch stage (e.g., the F1 stage204 of the processor pipeline 200 of FIG. 2) and the hit way may beencoded into the read index of the data array. The tag array may beaccessed in later fetch stage (e.g., the F2 stage 206 of the processorpipeline 200 of FIG. 2) and is only used for verifying the waypredictor.

The integrated circuit 110 includes an L1 data cache 152 for theprocessor core 120. For example, the L1 data cache 152 may be aset-associative VIPT cache, meaning that it is indexed purely withvirtual address bits VA[set] and tagged fully with all translatephysical address bits PA[msb:12]. For low power consumption, the tag anddata arrays may be looked up in serial so that at most a single dataSRAM way is accessed. For example, the line size of the L1 data cache152 may be 64 Bytes, and the beat size may be 16 Bytes.

The integrated circuit 110 includes an outer memory system 160, whichmay include memory storing instructions and data and/or provide accessto a memory 162 external to the integrated circuit that storesinstructions and/or data. For example, the outer memory system 160 mayinclude an L2 cache, which may be configured to implement a cachecoherency protocol/policy to maintain cache coherency across multiple L1caches. Although not shown in FIG. 1, the integrated circuit 110 mayinclude multiple processor cores in some implementations. For example,the outer memory system 160 may include multiple layers.

The integrated circuit 110 includes an immediate jump handler circuit170. The immediate jump handler circuit 170 may be configured to detecta sequence of instructions fetched by the processor core 120, whereinthe sequence of instructions includes a first instruction with a resultthat depends on an immediate field of the first instruction and aprogram counter value followed by a second instruction that is anindirect jump instruction. In some implementations, the processor core120 is configured to execute instructions of a RISC V instruction setand the first instruction is an AUIPC instruction and the secondinstruction is a JALR instruction. The immediate jump handler circuit170 may be configured to, responsive to detection of the sequence ofinstructions, prevent the indirect jump target predictor circuit fromgenerating a target address prediction for the second instruction. Theimmediate jump handler circuit 170 may be configured to, responsive todetection of the sequence of instructions, determine a target addressfor the second instruction before the first instruction is issued to anexecution stage of the pipeline. The immediate jump handler circuit 170may be configured to write the target address to a fetch target queuethat is configured to receive predictions from the indirect jump targetpredictor circuit 132. For example, the target address for the secondinstruction may be determined before the first instruction reaches adecode stage of the processor pipeline 130. For example, the immediatejump handler circuit 170 may include the immediate jump scanning circuit230 and the immediate jump determination circuit 232 of FIG. 2.

For example, the immediate jump handler circuit 170 may detect thesequence of instructions before they enter the fetch stage that includesthe indirect jump target predictor circuit 132. In some implementations,the processor pipeline 130 includes multiple fetch stages, the immediatejump handler circuit 170 detects the sequence of instructions as theypass through an early fetch stage that is earlier in the processorpipeline 130 than the fetch stage that includes the indirect jump targetpredictor circuit 132. The immediate jump handler circuit 170 may beconfigured to, responsive to detection of the sequence of instructions,disable the indirect jump target predictor circuit 132. For example, theimmediate jump handler circuit 170 may be configured to update a statusbit in an instruction cache tag, which causes the indirect jump targetpredictor circuit 132 to be disabled when the second instruction entersthe fetch stage of the pipeline that includes the indirect jump targetpredictor circuit 132. For example, the immediate jump handler circuit170 may be configured to update a status bit in an instruction cache waypredictor, which causes the indirect jump target predictor circuit 132to be disabled when the second instruction enters the fetch stage of thepipeline that includes the indirect jump target predictor circuit 132.

For example, the immediate jump handler circuit 170 may be configured todetect the sequence of instructions by scanning values stored in acacheline of the L1 instruction cache 150. In some implementations, theimmediate jump handler circuit 170 is configured to detect the sequenceof instructions by scanning values appearing on a memory bus asinstructions are being input to the L1 instruction cache 150 via thememory bus.

In some implementations, the immediate jump handler circuit 170 isconfigured to: detect a sequence of instructions fetched by theprocessor core 120, wherein the sequence of instructions includes anAUIPC instruction followed by a JALR instruction; responsive todetection of the sequence of instructions, disable the indirect jumptarget predictor circuit 132 to prevent the indirect jump targetpredictor circuit 132 from generating a target address prediction forthe JALR instruction; responsive to detection of the sequence ofinstructions, determine a target address for the JALR instruction beforethe AUIPC instruction is issued to an execution stage of the pipeline130; and write the target address to the fetch target queue in an entrycorresponding to the JALR instruction.

The integrated circuit 110 includes an indirect jump detector circuit180 configured to: check a cacheline for an indirect jump instruction byscanning values appearing on a memory bus as the cacheline is beinginput to an instruction cache via the memory bus; update, based on thecheck, a hint bit associated with the cacheline to indicate absence ofthe indirect jump instruction in the cacheline; and, based on the hintbit, disable the indirect jump target predictor circuit 132 to preventthe indirect jump target predictor circuit 132 from generating a targetaddress prediction when instructions of the cacheline enter a stage ofthe pipeline that includes the indirect jump target predictor circuit132. For example, the indirect jump instruction may be a JALRinstruction of a RISC V instruction set. For example, the hint bit maybe stored in an instruction cache way predictor (e.g., in the L1instruction cache 150). For example, the hint bit may be stored in aninstruction cache tag (e.g., in the L1 instruction cache 150). Theindirect jump detector circuit 180 may serve to save power by disablingthe indirect jump target predictor circuit 132 at times when no indirectjump instruction is being fetched. For example, the indirect jumpdetector circuit 180 may be configured to implement the process 600 ofFIG. 6.

FIG. 2 is a block diagram of an example of a portion of a processorpipeline 200 for executing instructions using fetch stage handling ofindirect jumps. The processor pipeline 200 includes multiple fetchstages: an F0 stage 202, an F1 stage 204, an F2 stage 206, and an F3stage 208. The processor pipeline 200 includes a decode stage 210following the fetch stages 202 through 208. Although not shown in FIG.2, the processor pipeline 200 may include additional stages, such as,rename, dispatch, issue, execute, memory access, and write-back stages.

The processor pipeline 200 includes an indirect jump target predictorcircuit 220 in the F3 stage 208 of the pipeline 200 configured togenerate predictions for target addresses of fetched indirect jumpinstructions. The processor pipeline 200 includes a fetch target queue222 for storing target address predictions from the indirect jump targetpredictor circuit 220 for use in later stages of the pipeline 200. Theindirect jump target predictor circuit 220 is a structure used topredict the target of indirect jump instructions (e.g., RISC-V JALRinstructions). The encoding of the source register and destinationregister fields of the indirect jump instruction may provide hints as tothe usage of the indirect jump instruction as a function call or return.In some implementations, the indirect jump target predictor circuit 220does not predict the targets of function returns, instead a ReturnAddress Stack (RAS) is used. For example, the indirect jump targetpredictor circuit 220 may be an ITTAGE-style predictor, which is similarin design to a branch direction predictor (BDP). However, as opposed topredicting branch direction, the indirect jump target predictor circuit220 provides target addresses. For example, the indirect jump targetpredictor circuit 220 may be SRAM-based, and to be more area efficientmay be designed to use single-ported memories. In some implementations,there is no structural hazard between prediction and updates on theindirect jump target predictor circuit 220.

As an area optimization, it may be observed that it is likely theindirect jump target predictor circuit 220 will only need to reference asmall number of memory ranges within a given window of time. Theindirect jump target predictor circuit 220 may use a level ofindirection to compress the storage of the upper target virtual addressbits. In some implementations, each entry in the indirect jump targetpredictor circuit 220 may therefore only keep a certain number of lowerbits and a reference to a table containing the upper bits. This table isreferred to as a High Array.

For example, the indirect jump target predictor circuit 220 may maintaina table with respective entries that include: an index into a HighArray, which stores upper target bits; low bits of target programcounter (PC); and a tag, which may be a hashed tag. Each entry in theindirect jump target predictor circuit 220 may also have a counter(e.g., 1 bit or 2 bits), which is used for indicating the usefulness ofeach entry and affects the replacement policy. These counter bits arestored in flop arrays.

To avoid needing to store the resolved target of each indirect jumpinstruction (e.g., JALR) in a branch resolution queue, the indirect jumptarget predictor circuit 220 may be updated directly after the branchunit resolves the jump instead of at retirement. When an indirect jumptarget predictor circuit 220 jump issues to the branch unit, the branchresolution queue index is sent back to the branch resolution queue andthe indirect jump target predictor circuit 220 prediction information(e.g., counter bits and provider table index) are read out of the branchresolution queue. When the indirect jump instruction is in a write backstage, an update request may be sent to the indirect jump targetpredictor circuit 220. For example, the update pipeline therefore may beas follows: at an issue stage, a branch unit sends the branch resolutionqueue index back to the branch resolution queue; at a register readstage, the indirect jump target predictor circuit 220 predictioninformation is read from the branch resolution queue; at an executionstage, the indirect jump target predictor circuit 220 update request isconstructed and flopped into write back; and, in a write back stage,send update request to the indirect jump target predictor circuit 220along with misprediction indication. The indirect jump target predictorcircuit 220 may re-compute table indices and tags, and CAM high arrayusing upper target bits.

If the indirect jump target predictor circuit 220 gets an update for acorrectly predicted jump, it may set the counter bit for the providerentry. If the target was mispredicted, the indirect jump targetpredictor circuit 220 may update the provider entry if the counter bitwas zero, or decrement the counter bit if not. The indirect jump targetpredictor circuit 220 may also attempt to allocate into a higher tablethan the provider table. For example, starting from the nexthighest-indexed table, the counter bits may be scanned. If a table has acounter of zero, then the indirect jump target predictor circuit 220 mayallocate into that table. If all counter bits are set, then a failedallocation may be signaled. A saturating counter may be incremented on afailed allocation, and decremented on a successful allocation.Saturation of the counter indicates trouble installing new entries intothe indirect jump target predictor circuit 220 due to long-livedentries. If saturation occurs, the counter bit arrays for all entries ofthe indirect jump target predictor circuit 220 may be flash cleared sothat new useful entries may be installed. In some implementations, eachentry of the indirect jump target predictor circuit 220 only stores aportion of the target address. When allocating into the indirect jumptarget predictor circuit 220, the High Array may be CAM'd with the upperbits of the resolved target. If a matching entry is found, the index ofthat entry may be written to the hiIdx field of the entry in the jumptarget predictor circuit 220. If no matching entry is found, an entry ofthe High Array is allocated according to a Pseudo-LRU replacementpolicy, and this index is written to the hiIdx field.

The processor pipeline 200 includes an immediate jump handler circuit,including an immediate jump scanning circuit 230 and an immediate jumpdetermination circuit 232. The immediate jump scanning circuit 230 maybe configured to detect sequences if instructions that form an indirectjump with a target address that can be determined based on informationavailable in the fetch stages. The sequence of instructions includes afirst instruction, with a result that depends on an immediate field ofthe first instruction and a program counter value, followed by a secondinstruction that is an indirect jump instruction. For example, in RISC-Vprocessor core, the sequence of instructions may include an AUIPCinstruction followed by a JALR instruction. The immediate jump scanningcircuit 230 is configured to detect the sequence of instructions byscanning values appearing on a memory bus from the memory bus interface240 as instructions are being input to an L1 instruction cache 250 viathe memory bus. Upon detecting the sequence of instructions, theimmediate jump scanning circuit 230 may update a status bit in aninstruction cache way predictor 252 to indicate that a cachelineassociated with the status bit includes the sequence of instructions.Updating the status bit may cause the indirect jump target predictorcircuit to be disabled when the second instruction enters the F3 stage208 of the pipeline 200 that includes the indirect jump target predictorcircuit 220.

When this cacheline is later read out from the L1 instruction cache 250in the F2 stage 206, the value of this status bit may be passed via apipeline register as a immediate jump hint to enable input of theindirect jump target predictor circuit 220 that will be available intime for use at the F3 stage 208, which may save power by preventing theindirect jump target predictor circuit 220 from running to generate atarget address prediction for an indirect jump instruction of thesequence of instructions. Thus, the immediate jump scanning circuit 230detects the sequence of instructions before they enter the F3 stage 208that includes the indirect jump target predictor circuit 220. Theimmediate jump scanning circuit 230 is configured to, responsive todetection of the sequence of instructions, disable the indirect jumppredictor by passing of the immediate jump hint stored in the status bitof the instruction cache way predictor 252 for use in an enable input ofthe indirect jump target predictor circuit 220 when the correspondingcacheline is read out of the L1 instruction cache 250.

After the cacheline is read out of the L1 instruction cache 250, thecacheline may be rotated in the F3 stage 208 to access relevantinstructions, which may be input to an instruction queue 260 that holdsinstructions for decode and also input to the immediate jumpdetermination circuit 232. The immediate jump determination circuit 232is configured to detect the sequence of instructions and determine atarget address for the indirect jump instruction of the sequence ofinstructions based on an immediate and a program counter value of thesequence of instructions. The processor pipeline 200 includes amultiplexor 270 that is used to select the target address determined bythe immediate jump determination circuit 232 and write the targetaddress to the fetch target queue 222 in lieu of a target addressprediction from the indirect jump target predictor circuit 220 for theindirect jump instruction of the sequence of instructions.

FIG. 3 is a memory map of an example of a sequence of instructions 300that includes a first instruction 310 with a result that depends on animmediate field of the first instruction and a program counter valuefollowed by a second instruction 320 that is an indirect jumpinstruction. The first instruction 310 includes an opcode 312, adestination register field 314 that identifies an architectural registerto be used to store a result of the first instruction 310, and animmediate 316 that is to be combined with (e.g., added to) a programcounter value to determine the result of the first instruction. Thesecond instruction 320 includes an opcode 322, a source register field324 that identifies an architectural register to be accessed, and animmediate 326 that is to be combined with (e.g., added to) a valuestored in the source register 324 to determine a target address of thesecond instruction. For example, in a RISC-V processor core, the firstinstruction may be an AUIPC instruction and the second instruction maybe a JALR instruction.

In some implementations, the first instruction 310 is adjacent to thesecond instruction 320 in memory and thus the second instruction 320immediately follows the first instruction 310. In some implementations,there may be one or more additional intervening instructions stored inmemory locations between the first instruction 310 and the secondinstruction 320 and thus the second instruction 320 follows the firstinstruction 310, but does not immediately follow the first instruction310. Where the one or more intervening instructions do not write todestination register 314 before it is accessed as the source register324, the sequence of instructions 300 may still function as an immediatejump, for which the target address may be determined during a fetchstage of a processor pipeline (e.g., the processor pipeline 130).

FIG. 4 is a flow chart of an example of a process 400 for fetch stagehandling of indirect jumps. The process 400 includes detecting 450 asequence of instructions fetched by a processor core that includes afirst instruction, with a result that depends on an immediate field ofthe first instruction and a program counter value, followed by a secondinstruction that is an indirect jump instruction; responsive todetection of the sequence of instructions, preventing 420 an indirectjump target predictor circuit from generating a target addressprediction for the second instruction; responsive to detection of thesequence of instructions, determining a target address for the secondinstruction before the first instruction is issued; and writing 440 thetarget address to a fetch target queue. The process 400 may provideadvantages over conventional techniques, such as, for example, avoidingmispredictions of target addresses and resulting pollution of anindirect jump predictor and performance degradation, and/or conservationof power consumption. For example, the process 400 may be implementedusing the integrated circuit 110 of FIG. 1. For example, the process 400may be implemented using the processor pipeline 200 of FIG. 2.

The process 400 includes detecting 410 a sequence of instructionsfetched by a processor core (e.g., the processor core 120). The sequenceof instructions includes a first instruction, with a result that dependson an immediate field of the first instruction and a program countervalue, followed by a second instruction that is an indirect jumpinstruction. For example, the processor core may be configured toexecute instructions of a RISC V instruction set and the firstinstruction is an AUIPC instruction and the second instruction is a JALRinstruction. In some implementations, detecting 410 the sequence ofinstructions fetched by the processor core includes detecting thesequence of instructions by scanning values appearing on a memory bus asinstructions are being input to an instruction cache (e.g., the L1instruction cache 250) via the memory bus. In some implementations,detecting 410 the sequence of instructions fetched by the processor coreincludes detecting the sequence of instructions by scanning valuesstored in a cacheline of an instruction cache. For example, the sequenceof instructions may be detected 410 before they enter a fetch stage(e.g., the F3 stage 208 of the processor pipeline 200) that includes anindirect jump target predictor circuit (e.g., the indirect jump targetpredictor circuit 220). In some implementations, the pipeline includesmultiple fetch stages, the sequence of instructions is detected as theypass through an early fetch stage (e.g., the F0 stage 202 of theprocessor pipeline 200) that is earlier in the pipeline than a fetchstage (e.g., the F3 stage 208 of the processor pipeline 200) thatincludes the indirect jump target predictor circuit.

The process 400 includes, responsive to detection 410 of the sequence ofinstructions, preventing 420 an indirect jump target predictor circuit(e.g., the indirect jump target predictor circuit 132) from generating atarget address prediction for the second instruction. For example,preventing 420 the indirect jump target predictor circuit fromgenerating a target address prediction for the second instruction mayinclude, responsive to detection 410 of the sequence of instructions,disabling the indirect jump predictor. In some implementations,preventing 420 the indirect jump target predictor circuit fromgenerating a target address prediction for the second instructionincludes updating a status bit in an instruction cache tag, which causesthe indirect jump target predictor circuit to be disabled when thesecond instruction enters a stage of the pipeline (e.g., the F3 stage208 of the processor pipeline 200) that includes the indirect jumptarget predictor circuit. In some implementations, preventing 420 theindirect jump target predictor circuit from generating a target addressprediction for the second instruction includes updating a status bit inan instruction cache way predictor (e.g., the instruction cache waypredictor 252), which causes the indirect jump target predictor circuitto be disabled when the second instruction enters a stage of thepipeline that includes the indirect jump target predictor circuit.

The process 400 includes, responsive to detection 410 of the sequence ofinstructions, determining 430 a target address for the secondinstruction before the first instruction is issued to an execution stageof a pipeline of the processor core. For example, the target address forthe second instruction may be determined 430 before the firstinstruction reaches a decode stage of the pipeline (e.g., the processorpipeline 130).

The process 400 includes writing 440 the target address to a fetchtarget queue (e.g., the fetch target queue 222) that is configured toreceive predictions from the indirect jump target predictor circuit. Forexample, a multiplexor (e.g., the multiplexor 270) may be used to selectthe target address determined 430 rather than a target addressprediction from the indirect jump target predictor circuit.

Although not shown in FIG. 4, the process 400 may be employed incombination with the process 600 of FIG. 6 to further reduce powerconsumption in an indirect jump target predictor circuit. For example,the process 400 may further include: checking 610 a cacheline for theindirect jump instruction by scanning values appearing on a memory busas the cacheline is being input to an instruction cache via the memorybus; updating 630, based on the check, a hint bit associated with thecacheline to indicate absence of the indirect jump instruction in thecacheline; and, based on the hint bit, disabling 660 an indirect jumptarget predictor circuit to prevent the indirect jump target predictorcircuit from generating a target address prediction when instructions ofthe cacheline enter a stage of the pipeline that includes the indirectjump target predictor circuit.

FIG. 5 is a flow chart of an example of a process 500 for determining atarget address for an indirect jump instruction that depends on aprogram counter and one or more immediates of a sequence ofinstructions. The process 500 includes left shifting 510 an immediate(e.g., the immediate 316) of a first instruction (e.g., the firstinstruction 310); adding 520 the shifted immediate of the firstinstruction to an immediate (e.g., the immediate 326) of a secondinstruction (e.g., the second instruction 320); and adding 530 the sumof the immediates to a program counter value to obtain a target address.For example, the first instruction may be a RISC V AUIPC instruction andthe second instruction may be a RISC-V JALR instruction. For example,the immediate of the first instruction may be left shifted 510 by anumber of bits equal to the size of the immediate of the secondinstruction. In some implementations, the number of the bits of theimmediate of the first instruction and the number of bits of theimmediate of the second instruction together equal the number of bits ofan architectural register of a processor core implementing the process500. The process 500 may be implemented by a logic circuit of a fetchstage with access to the first instruction and the second instruction asthey are stored in a buffer. The steps of the process 500 may beperformed in various orders or simultaneously. For example, the shiftedunsigned immediate of the first instruction may be added to the programcounter value before the immediate of the second instruction is added tothe result to obtain the target address. In some implementations (notshown in FIG. 5), the immediate of the second instruction, rather thanthe immediate of the first instruction, is left shifted before beingadded in. For example, the process 500 may be implemented using theintegrated circuit 110 of FIG. 1. For example, the process 500 may beimplemented using the processor pipeline 200 of FIG. 2.

FIG. 6 is a flow chart of an example of a process 600 for selectivelydisabling an indirect jump target predictor circuit in the absence ofindirect jumps. The process 600 includes checking 610 a cacheline for anindirect jump instruction as the cacheline is being input to aninstruction cache via the memory bus; if the check detects an indirectjump instruction in the cacheline, updating 620, based on the check, ahint bit associated with the cacheline to indicate presence of theindirect jump instruction in the cacheline; if the check does not detectan indirect jump instruction in the cacheline, updating 630, based onthe check, a hint bit associated with the cacheline to indicate absenceof the indirect jump instruction in the cacheline; at some later time,reading 640 the cacheline from the cache into a fetch stage of aprocessor pipeline; if the hint bit indicates presence of an indirectjump instruction in the cacheline, based on the hint bit, enabling 650an indirect jump target predictor circuit to allow the indirect jumptarget predictor circuit to generate a target address prediction wheninstructions of the cacheline enter a stage of the pipeline thatincludes the indirect jump target predictor circuit; and, if the hintbit indicates absence of an indirect jump instruction in the cacheline,based on the hint bit, disabling 660 an indirect jump target predictorcircuit to prevent the indirect jump target predictor circuit fromgenerating a target address prediction when instructions of thecacheline enter a stage of the pipeline that includes the indirect jumptarget predictor circuit. For example, the process 600 may beimplemented using the integrated circuit 110 of FIG. 1.

The process 600 includes checking 610 a cacheline for an indirect jumpinstruction by scanning values appearing on a memory bus as thecacheline is being input to an instruction cache (e.g., the L1instruction cache 150) via the memory bus. For example, the indirectjump instruction is a JALR instruction of a RISC V instruction set. Insome cases, an indirect jump instruction appears entirely within asingle cacheline and checking 610 the cacheline includes detecting thecomplete indirect jump instruction in the cacheline as it is transferredinto the cache. For example, a JALR instruction may be recognized bydetecting an opcode within the lower 16 bits of the instruction. In somecases, an indirect jump instruction may cross a cacheline boundary. Forexample, a lower portion of the instruction may be in a first cachelineand higher portion of the instruction may be in a second cacheline. Theorder in which these two cachelines is received in the cache may not beguaranteed, which could further complicate checking 610 for the presenceof an indirect jump instruction in a cacheline. Special logic may beemployed to attempt to check 610 whether an indirect jump instruction(e.g., a JALR) ends in the cacheline being loaded into the cache.

For example, when supporting the C extension of the RISC-V instructionset, it's possible for 32-bit JALR instructions to cross cachelines. Asa power optimization, a way predictor may store a hint bit indicatingthat a JALR instruction likely ends in this cacheline. While fetching,an indirect jump target predictor circuit (e.g., the indirect jumptarget predictor circuit 132) may only be accessed if the hint bit isset to indicate the presence of a JALR instruction in a cacheline thatis being fetched. To generate this hint bit, a miss queue of a cache(e.g., the L1 instruction cache 150) may have some extra logic to scanincoming fill data and detect when JALR instructions may end in thiscacheline. For example, parentValid, parentFilled, parent (e.g., apointer to a miss queue entry for a parent cacheline), and jalrCrossentry fields may be used for this purpose. A common scenario is that thefetch unit will generate a cache miss, and then a few sequentialprefetches. When a miss queue entry is allocated, the miss queue willcheck to see if the previously allocated entry is still valid. If so theparentValid field is set to 1 and the parent field is set to the indexof the previously allocated entry. The previously allocated entry isreferred to as the “parent” entry. If the parent entry fills first, thenthe parentFilled field is set to one, and the jalrCross field is set toone if the last 16 bits of the parent fill data looks like the lower 16bits of a 32-bit JALR. When the fill data for an entry comes back, eachbeat of fill data is also scanned for potential JALR instructions. Thisis tricky when supporting the C extension because it may not be possibleto know if the first 16 bits of the cache block corresponds to thesecond half of a 32-bit instruction or not. So, both cases may beassumed. When an entry fills, the hint bit is set if any of thefollowing cases is true: (1) When the miss request was made, the fetchpipeline already had the first 16-bits of an RVI instruction and itlooks like a JALR. (2) The parent entry was valid and filled first, andthe jalrCross bit is set. (3) When this entry's fill data was scanned,we might have a complete JALR instruction.

If (at step 615) an indirect jump instruction ending in the cachelinehas been detected, then the process 600 includes updating 620, based onthe check 610, a hint bit associated with the cacheline to indicatepresence of the indirect jump instruction in the cacheline. If (at step615) an indirect jump instruction ending in the cacheline has not beendetected, then the process 600 includes updating 630, based on the check610, a hint bit associated with the cacheline to indicate absence of theindirect jump instruction in the cacheline. In some implementations, thehint bit is stored in an instruction cache way predictor (e.g., theinstruction cache way predictor 252). In some implementations, the hintbit is stored in an instruction cache tag (e.g., the in the L1instruction cache 250).

The process 600 includes reading 640 the cacheline from the cache into afetch stage of a processor pipeline (e.g., the processor pipeline 130).For example, the cacheline may be read 640 out of the cache and rotatedas needed before placing instructions of the cacheline in an instructionqueue (e.g., the instruction queue 260) for decode.

If (at step 645) the hint bit indicates the presence of an indirect jumpinstruction ending in the cacheline, then the process 600 includes,based on the hint bit, enabling 650 an indirect jump target predictorcircuit (e.g., the indirect jump target predictor circuit 132) to allowthe indirect jump target predictor circuit to generate a target addressprediction when instructions of the cacheline enter a stage of thepipeline that includes the indirect jump target predictor circuit. If(at step 645) the hint bit indicates the absence of an indirect jumpinstruction ending in the cacheline, then the process 600 includes,based on the hint bit, disabling 660 the indirect jump target predictorcircuit to prevent the indirect jump target predictor circuit fromgenerating a target address prediction when instructions of thecacheline enter a stage of the pipeline that includes the indirect jumptarget predictor circuit.

Misprediction or errors in the IJTP hit bit may occur and may need to becorrected. For example, when supporting the C extension in a RISC-Vprocessor, it may not be possible to precisely determine when a cacheblock starts with the second half of a JALR instruction at the time offill. With multiple misses outstanding, fills may return out of order.If the fetch pipeline detects a JALR instruction while searching forbranches and jumps in a later fetch stage of a processor pipeline (e.g.,the F3 stage 208 of the processor pipeline 200), but the hint bit readout of the way predictor indicated no JALR instructions, then there wasa misprediction. In this case the indirect jump target predictor circuitwas disabled and not accessed and there is no valid prediction for thatfetch group. In some implementations, this misprediction is handled bytreating this as a way predictor misprediction, correcting the UTP hintbit (e.g., in the way predictor), and re-fetching. For example, handlinga missed indirect jump instruction as a way predictor misprediction mayincur a performance penalty (e.g., a 4-cycle penalty), but is expectedto be rare.

In a first aspect, the subject matter described in this specificationcan be embodied in an integrated circuit for executing instructions thatincludes a processor core including a pipeline configured to executeinstructions, an indirect jump target predictor circuit in a fetch stageof the pipeline configured to generate predictions for target addressesof fetched indirect jump instructions, and an immediate jump handlercircuit configured to: detect a sequence of instructions fetched by theprocessor core, wherein the sequence of instructions includes a firstinstruction, with a result that depends on an immediate field of thefirst instruction and a program counter value, followed by a secondinstruction that is an indirect jump instruction; responsive todetection of the sequence of instructions, prevent the indirect jumptarget predictor circuit from generating a target address prediction forthe second instruction; and, responsive to detection of the sequence ofinstructions, determine a target address for the second instructionbefore the first instruction is issued to an execution stage of thepipeline.

In a second aspect, the subject matter described in this specificationcan be embodied in methods that include detecting a sequence ofinstructions fetched by a processor core, wherein the sequence ofinstructions includes a first instruction, with a result that depends onan immediate field of the first instruction and a program counter value,followed by a second instruction that is an indirect jump instruction;responsive to detection of the sequence of instructions, preventing anindirect jump target predictor circuit from generating a target addressprediction for the second instruction; and, responsive to detection ofthe sequence of instructions, determining a target address for thesecond instruction before the first instruction is issued to anexecution stage of a pipeline of the processor core.

In a third aspect, the subject matter described in this specificationcan be embodied in an integrated circuit for executing instructions thatincludes a processor core including a pipeline configured to executeinstructions of a RISC V instruction set, an indirect jump targetpredictor circuit in a fetch stage of the pipeline configured togenerate predictions for target addresses of fetched indirect jumpinstructions and output the predictions to a fetch target queue, and animmediate jump handler circuit configured to: detect a sequence ofinstructions fetched by the processor core, wherein the sequence ofinstructions includes an AUIPC instruction followed by a JALRinstruction; responsive to detection of the sequence of instructions,disable the indirect jump target predictor circuit to prevent theindirect jump target predictor circuit from generating a target addressprediction for the JALR instruction; responsive to detection of thesequence of instructions, determine a target address for the JALRinstruction before the AUIPC instruction is issued to an execution stageof the pipeline; and write the target address to the fetch target queuein an entry corresponding to the JALR instruction.

In a fourth aspect, the subject matter described in this specificationcan be embodied in methods that include checking a cacheline for anindirect jump instruction by scanning values appearing on a memory busas the cacheline is being input to an instruction cache via the memorybus; updating, based on the check, a hint bit associated with thecacheline to indicate absence of the indirect jump instruction in thecacheline; and, based on the hint bit, disabling an indirect jump targetpredictor circuit to prevent the indirect jump target predictor circuitfrom generating a target address prediction when instructions of thecacheline enter a stage of a processor pipeline that includes theindirect jump target predictor circuit.

In a fifth aspect, the subject matter described in this specificationcan be embodied in an integrated circuit for executing instructions thatincludes a processor core including a pipeline configured to executeinstructions, an indirect jump target predictor circuit in a fetch stageof the pipeline configured to generate predictions for target addressesof fetched indirect jump instructions, and an indirect jump detectorcircuit configured to: check a cacheline for an indirect jumpinstruction by scanning values appearing on a memory bus as thecacheline is being input to an instruction cache via the memory bus;update, based on the check, a hint bit associated with the cacheline toindicate absence of the indirect jump instruction in the cacheline; and,based on the hint bit, disable the indirect jump target predictorcircuit to prevent the indirect jump target predictor circuit fromgenerating a target address prediction when instructions of thecacheline enter a stage of the pipeline that includes the indirect jumptarget predictor circuit.

While the disclosure has been described in connection with certainembodiments, it is to be understood that the disclosure is not to belimited to the disclosed embodiments but, on the contrary, is intendedto cover various modifications and equivalent arrangements includedwithin the scope of the appended claims, which scope is to be accordedthe broadest interpretation so as to encompass all such modificationsand equivalent structures.

1. An integrated circuit for executing instructions comprising: aprocessor core including a pipeline configured to execute instructions;an indirect jump target predictor circuit in a fetch stage of thepipeline configured to generate predictions for target addresses offetched indirect jump instructions; and an immediate jump handlercircuit configured to: detect a sequence of instructions fetched by theprocessor core in a fetch stage of the pipeline, wherein the sequence ofinstructions includes a first instruction, with a result that depends onan immediate field of the first instruction and a program counter value,followed by a second instruction that is an indirect jump instruction;responsive to the detection of the sequence of instructions, prevent theindirect jump target predictor circuit from generating a target addressprediction for the second instruction; and responsive to the detectionof the sequence of instructions, determine, based on the immediate fieldof the first instruction and the program counter value, a target addressfor the second instruction before the first instruction is issued to anexecution stage of the pipeline.
 2. The integrated circuit of claim 1,in which the immediate jump handler circuit detects the sequence ofinstructions before they enter the fetch stage that includes theindirect jump target predictor circuit, and the immediate jump handlercircuit is configured to: responsive to detection of the sequence ofinstructions, disable the indirect jump target predictor circuit.
 3. Theintegrated circuit of claim 1, in which the pipeline includes multiplefetch stages, the immediate jump handler circuit detects the sequence ofinstructions as they pass through an early fetch stage that is earlierin the pipeline than the fetch stage that includes the indirect jumptarget predictor circuit, and the immediate jump handler circuit isconfigured to: responsive to detection of the sequence of instructions,disable the indirect jump target predictor circuit.
 4. The integratedcircuit of claim 1, in which the immediate jump handler circuit isconfigured to update a status bit in an instruction cache tag, whichcauses the indirect jump target predictor circuit to be disabled whenthe second instruction enters the fetch stage of the pipeline thatincludes the indirect jump target predictor circuit.
 5. The integratedcircuit of claim 1, in which the immediate jump handler circuit isconfigured to update a status bit in an instruction cache way predictor,which causes the indirect jump target predictor circuit to be disabledwhen the second instruction enters the fetch stage of the pipeline thatincludes the indirect jump target predictor circuit.
 6. The integratedcircuit of claim 1, in which the immediate jump handler circuit isconfigured to detect the sequence of instructions by scanning valuesappearing on a memory bus as instructions are being input to aninstruction cache via the memory bus.
 7. The integrated circuit of claim1, in which the immediate jump handler circuit is configured to detectthe sequence of instructions by scanning values stored in a cacheline ofan instruction cache.
 8. The integrated circuit of claim 1, in which theimmediate jump handler circuit is configured to write the target addressto a fetch target queue that is configured to receive predictions fromthe indirect jump target predictor circuit.
 9. The integrated circuit ofclaim 1, in which the target address for the second instruction isdetermined before the first instruction reaches a decode stage of thepipeline.
 10. The integrated circuit of claim 1, in which the processorcore is configured to execute instructions of a RISC V instruction setand the first instruction is an AUIPC instruction and the secondinstruction is a JALR instruction.
 11. A method comprising: detecting asequence of instructions fetched by a processor core in a fetch stage ofa pipeline, wherein the sequence of instructions includes a firstinstruction, with a result that depends on an immediate field of thefirst instruction and a program counter value, followed by a secondinstruction that is an indirect jump instruction; responsive to thedetection of the sequence of instructions, preventing an indirect jumptarget predictor circuit from generating a target address prediction forthe second instruction; and responsive to the detection of the sequenceof instructions, determining, based on the immediate field of the firstinstruction and the program counter value, a target address for thesecond instruction before the first instruction is issued to anexecution stage of a pipeline of the processor core.
 12. The method ofclaim 11, in which the sequence of instructions is detected before theyenter a fetch stage that includes the indirect jump target predictorcircuit, and in which preventing the indirect jump target predictorcircuit from generating a target address prediction for the secondinstruction comprises: responsive to detection of the sequence ofinstructions, disabling the indirect jump target predictor circuit. 13.The method of claim 11, in which the pipeline includes multiple fetchstages, the sequence of instructions is detected as they pass through anearly fetch stage that is earlier in the pipeline than a fetch stagethat includes the indirect jump target predictor circuit, and in whichpreventing the indirect jump target predictor circuit from generating atarget address prediction for the second instruction comprises:responsive to detection of the sequence of instructions, disabling theindirect jump target predictor circuit.
 14. The method of claim 11, inwhich preventing the indirect jump target predictor circuit fromgenerating a target address prediction for the second instructioncomprises: updating a status bit in an instruction cache tag, whichcauses the indirect jump target predictor circuit to be disabled whenthe second instruction enters a stage of the pipeline that includes theindirect jump target predictor circuit.
 15. The method of claim 11, inwhich preventing the indirect jump target predictor circuit fromgenerating a target address prediction for the second instructioncomprises: updating a status bit in an instruction cache way predictor,which causes the indirect jump target predictor circuit to be disabledwhen the second instruction enters a stage of the pipeline that includesthe indirect jump target predictor circuit.
 16. The method of claim 11,in which the detecting the sequence of instructions fetched by theprocessor core comprises: detecting the sequence of instructions byscanning values appearing on a memory bus as instructions are beinginput to an instruction cache via the memory bus.
 17. The method ofclaim 11, comprising: writing the target address to a fetch target queuethat is configured to receive predictions from the indirect jump targetpredictor circuit.
 18. The method of claim 11, comprising: checking acacheline for the indirect jump instruction by scanning values appearingon a memory bus as the cacheline is being input to an instruction cachevia the memory bus; updating, based on the check, a hint bit associatedwith the cacheline to indicate absence of the indirect jump instructionin the cacheline; and based on the hint bit, disabling an indirect jumptarget predictor circuit to prevent the indirect jump target predictorcircuit from generating a target address prediction when instructions ofthe cacheline enter a stage of the pipeline that includes the indirectjump target predictor circuit.
 19. An integrated circuit for executinginstructions comprising: a processor core including a pipelineconfigured to execute instructions of a RISC V instruction set; anindirect jump target predictor circuit in a fetch stage of the pipelineconfigured to generate predictions for target addresses of fetchedindirect jump instructions and output the predictions to a fetch targetqueue; and an immediate jump handler circuit configured to: detect asequence of instructions fetched by the processor core in a fetch stageof the pipeline, wherein the sequence of instructions includes an AUIPCinstruction followed by a JALR instruction; responsive to the detectionof the sequence of instructions, disable the indirect jump targetpredictor circuit to prevent the indirect jump target predictor circuitfrom generating a target address prediction for the JALR instruction;responsive to the detection of the sequence of instructions, determine,based on an immediate field of the AUIPC instruction and a programcounter value, a target address for the JALR instruction before theAUIPC instruction is issued to an execution stage of the pipeline; andwrite the target address to the fetch target queue in an entrycorresponding to the JALR instruction.
 20. The integrated circuit ofclaim 19, in which the immediate jump handler circuit is configured toupdate a status bit in an instruction cache way predictor, which causesthe indirect jump target predictor circuit to be disabled when the JALRinstruction enters the fetch stage of the pipeline that includes theindirect jump target predictor circuit.